The present invention relates to a method for producing silicon applicable to a semiconductor or photovoltaic power generation from a silane. More specifically, the present invention relates to a production method of high purity silicon which comprises removing and recovering silicon deposited on a substrate in a silicon deposition reactor from the substrate without cooling the silicon and with small effort so as to improve a rate of operation of the reactor.
A variety of methods for producing silicon to be used as a raw material for a semiconductor or photovoltaic system have heretofore been known, and some of them are already actually used in industry.
One of the actually used methods is a method called as a Siemens method. This method is a method in which a thin silicon filament which serves as a substrate for deposition is disposed in a bell jar and then heated by energization and trichlorosilane (SiHCl3) or monosilane (SiH4) is then brought into contact with the substrate so as to cause silicon to be deposited in solid form. The method is the most generally practiced method at present.
In the Siemens method, after termination of the energization, the bell jar is opened after the silicon filament which is a deposit is allowed to fully cool down, and after the fragile deposit is carefully recovered, another thin silicon filament must be disposed with high accuracy. Therefore, considerable effort is required each time the deposit is recovered, and a time interval between depositions is long, so that a rate of operation of the deposition reactor is low.
Meanwhile, methods for continuously recovering silicon deposited in a deposition reactor are proposed in JP-A 59-121109, JP-A 51-37819 and JP-A 2002-29726. These are methods in which while silicon is being deposited in a molten state on a surface of a substrate heated at least to the melting point of silicon by bringing a silane into contact with the surface of the substrate, the molten deposit is recovered from the surface of the substrate as it is and then extracted from the reactor as molten silicon or cooled/solidified silicon.
Since these methods carry out deposition of silicon at high temperatures, these methods exhibit very good deposition efficiency and can produce silicon at a low cost. However, molten silicon obtained by these methods has such strong reactivity that it is even called xe2x80x9csuper solventxe2x80x9d (universal solvent) and has a problem that it is liable to be contaminated by the surface of the substrate with which the molten silicon makes contact. Accordingly, it has been difficult in some cases to obtain high purity silicon which can be used particularly in semiconductors.
Therefore, an object of the present invention is to provide a silicon production method which can deposit and recover silicon usable in a semiconductor or photovoltaic power generation from a silane continuously, can improve a rate of operation of a reactor thereby, and can produce high purity silicon continuously as compared with a conventional silicon production method which causes silicon to be deposited in a molten state.
Other objects and advantages of the present invention will be apparent from the following description.
According to the present invention, the above object and advantage of the present invention can be achieved by repeating, in the same apparatus, a step of depositing silicon as a solid by heating a surface of a substrate to a temperature lower than the melting point of the silicon and a step of causing a portion or all of the deposited silicon to melt and drop by heating the surface of the substrate to a temperature equal to or higher than the melting point of the silicon when the deposition of the silicon in solid form has proceeded to a certain point.
That is, according to the present invention, there is provided a method for producing silicon which comprises a step (hereinafter referred to as xe2x80x9cstep 1xe2x80x9d) of depositing silicon on said surface by bringing a silane into contact with the surface of a substrate while the surface of the substrate is heated to and kept at a temperature lower than the melting point of the silicon and a step (hereinafter referred to as xe2x80x9cstep 2xe2x80x9d) of melting a portion or all of the deposited silicon and to drop and recover the melted silicon from the surface of the substrate by raising the temperature of the surface of the substrate.
In the above method of the present invention, by depositing silicon in solid form, the silicon is hardly contaminated by the substrate at the time of deposition of the silicon. Further, for recovery of the silicon, by causing at least silicon deposited on the surface of the substrate to melt and drop from the substrate, time during which the molten silicon is in contact with the surface of the substrate can be shortened as compared with a conventional method in which silicon remains in a molten state after deposited. As a result, contamination caused by contact between the substrate and the molten silicon can be reduced effectively. Further, according to the method of the present invention, deposition and recovery of silicon can be carried out continuously. In addition, since speed at which silicon is deposited on the surface of the substrate reaches a maximum in a temperature range slightly lower than the melting point of the silicon, a temperature range in which the deposition speed is high can be selected as compared with the conventional melt-deposition method, whereby productivity can be improved.